Since the introduction of the Sony Walkman in July of 1979, over 100 million units have been sold. The Oxford English Dictionary certified ‘walkman’ as a noun in 1986 describing it as a personal audio device. The recreational audio device has established itself as a mainstay for personal music enjoyment. Advances in the personal audio device market have typically been focused in two areas: size of the unit and headphone improvements. Headphones for personal audio systems have historically been air conduction systems that rely on tympanic hearing for sound transmission.
In tympanic hearing, sound travels through the ear canal to the eardrum making it vibrate. These vibrations are passed to three small bones in the middle ear, the ossicles, by a process called air conduction. These in turn pass the vibrations to the cochlea and the fluid it contains. Movement in this fluid bends the tiny hair cells along the length of the cochlea, generating signals in the auditory nerve. The nerve signals pass to the brain, which interprets them as sound. Bone conduction hearing is when sound vibrations are transmitted directly from the skull and jaw bones to the cochlea, missing the outer and middle ears. Air conduction sound systems provide stereo quality sound by taking advantage of the ability of the human brain to take in sound from the two ears and integrating the multiple sound sources into a single, richer sound. While bone conduction devices have traditionally been developed for the hearing impaired and as hearing aid devices until recently, these devices focused on transmitting sound in the speaking voice frequency range and have not been adapted for high fidelity musical signals. Additionally, the recreational audio systems for the underwater environment have traditionally relied on air conduction with ear plugs for the sound transmission.
While small, streamline systems exist for land based recreational audio, they are predominately of the air conduction type. Several of these systems have been waterproofed for use by swimmers. These systems rely on ear plugs that are placed in the ear such that an air bubble is formed in the ear canal. When this bubble is intact, the sound transmission is acceptable. However, the ear canal acoustic resonance is lost if it fills with water while the head is submerged. With bone conduction sound transmission, this disadvantage is overcome. Specifically, when the ear canal is filled with water, as when a swimmer is submerged, the mass of the water (4.5 times denser than air) acoustically loads the ear drum enhancing low frequency sound reception in the ear to bone conduction [Tonndorf, J. A New Concept of Bone Conduction, Arch Otol 87, 49-54 1968].
Common bone conduction type devices have been developed to transmit sound in the speech frequency range and have not been maximized to provide musical sound quality. In addition, bone conduction devices have been either large units that were heavy, bulky and uncomfortable for the user or have been devices integrated into a bite plate for sound transmission via the jaw bone (May U.S. Pat. No. 5,579,284). Bit plate type of sound transmission actually requires the user to continually bite down on the device in order to hear the sound.
An audio systems using bone conduction is shown in U.S. Pat. No. 4,791,673 to Schreiber. This invention is an audio listening system that includes both a bone conduction device and a sound source unit. The system has a transducer mounted in a c-shaped element that hooks around the ear of the user. A suction cup element is included as part of the transducer feature to ensure contact from the transducer to the mastoid region behind the ear of the user. This device is water resistant but not waterproof and has only one type of transducer to transmit sound to the user.
A further device is shown in U.S. Pat. No. 5,323,468 to Bottesch that provides a means for the conduction of sound waves through the mastoid bones of the user and selectively amplifying predetermined frequency ranges that the invention claims do not conduct well through the bone so as to maximize the transmission of all signals in the sound source frequency range. The invention is a small, light weight head gear that puts one or several transducers in contact with the mastoid region of the skull. The headgear is designed to provide stereophonic music to the user by transmitting the stereo sound signals separately to transducers located behind the ear of the user. This device is not waterproof and only provides one type of transducer for transmitting across the multiple frequency ranges.
A third bone conduction device is shown in U.S. Pat. No. 5,889,730 to May that provides an underwater audio communication system for transmitting voice through bone conduction at the mastoid region of the head. This device is designed to allow voice communication to and from an underwater user. The device mounts one or more of the same type transducers onto the users scuba face mask. A transceiver and amplifier is located on the back of the users head to transmit and receive ultrasonic sound signals for communication with the user.